Predicting the faith of exotic plant species in cold-climate mountains: our new paper is out now in early view! You can find it here.

Abisko, a small village north of the polar circle in Swedish Lapland. The origin of several mountain trails, winding through the pristine subarctic vegetation towards the breathtaking views at the top. A vegetation mostly consisting of slow-growing mosses and dwarf shrubs that seem to have been there forever. Yet during the last few years or decades, changes in this vegetation increasingly start to become apparent: several new species that are traditionally not a part of the subarctic vegetation are popping up along the trails. Clovers, common yarrow, sweetgrass or annual meadow grass, species that are typical residents of the milder parts of Europe, are now getting a foothold even here, in the high north. They border the trails, grow in the roadsides, line the buildings at the ski resorts and seem to follow humans with every step they take.

A typical subarctic mountain trail, winding through a blueberry field (Vaccinium myrtillus).

The higher up you go in the mountains, the less common these new species become. At some point, often close to the treeline, they disappear entirely. Why do they stop there? Are these elevations finally too cold for exotic species to survive, or is it just a matter of time till they find their way to higher elevations? What drives their distribution and, most importantly, where will we find them in the future?

The subarctic mountain vegetation is dominated by sturdy sedges, tiny mosses and small flowers, like this mountain avens (Dryas octopetala).

To find out what the chances of these non-native species are in the coldest and highest mountains, an international team of ecologists set up an experiment high above the current elevational limit of these species. This was done at 400 meter above the local treeline, in an area where spring and autumn are almost back to back, and the full growing season lasts only a little more than two months. In this harsh environment, dominated by sturdy sedges and tiny mosses, they tested the potential of six non-native species, which have their origins in the mild climate of Western Europe. They simulated different human stressors (e.g., disturbance and soil enrichment) and planted seeds of the six focal species along a temperature gradient. Expectations were low: for a species used to the mild winters and relatively long summers of Western Europe, the high alpine zone of subarctic Lapland was unlikely to be hospitable.

Experimental plot in the mountains of Swedish Lapland

Yet the results were unsettling. Under ideal circumstances (e.g. warm, south-facing slopes and disturbed plots), the seeded plants managed to survive two growing seasons and a winter and they also produced significantly more biomass than in the control treatments. In undisturbed natural vegetation, on the other hand, success of the sown exotic species was negligible. Thus, only plots in which anthropogenic influences, such as fertilization and removal of the vegetationere combined with a sufficiently warm microclimate, resulted in establishment success for the exotic species. If, and only if, those two factors were present, the possibilities for exotic species to establish increased drastically, even at elevations far above where we find them now.

Sweetgrass (Anthoxanthum odoratum) beating the odds in a disturbed plot on a south-facing slope.

Yet where to find warmth in these environments, you might ask? Surprisingly, the mountain climate is often drastically less harsh than one might expect when considering environmental heterogeneity occurring at very small spatial scales, even at high elevations. This is for a large part due to the complex topography of mountains: soils on south-facing slopes for example – which get a lot of direct sunlight all day long – can easily be 7 °C warmer than their surroundings. Yet humans themselves are creating such warm spots as well, by removing the alpine vegetation along trails, for example. The vegetation normally provides a buffer for extreme heat production at soil level during the summer, whereas removing it will increase the temperature. For tiny seedlings, these extra degrees might be lifesaving. In winter, on the other hand, a protective snow cover shelters the small plants from below-zero temperatures.

The surprisingly high success of exotic plants in warmer and more disturbed plots suggests that spread of exotic species into cold mountain regions is likely to increase significantly in the future, if climate keeps warming and anthropogenic disturbance of pristine alpine regions is not halted. But even now already, exotic species can make use of the warm spots in the landscape, either natural or augmented by humans, as stepping stones to reach higher elevations. The potential for exotic species invasion in cold environments is thus most likely heavily underestimated.

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Hallerbos 2017

The common bluebell (Hyacinthoides non-scripta), the signature flower of the Hallerbos

Single bluebell flower surviving on a wetter spot, as indicated by the field of wild garlic (Allium ursinum)

A really wet patch of forest, with giant horsetail (Equisetum telmateia) in a field of wild garlic (Allium ursinum)

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A bumblebee visiting yellow archangel (Lamium galeobdolon)

A bumblebee visiting yellow archangel (Lamium galeobdolon)

Wild garlic (Allium ursinum)

Wild garlic (Allium ursinum)

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Weirdly beautiful, the inflorescence of pendulous sedge (Carex pendula), typical for the wettest spots in the forest

A little stream in the Hallerbos, surrounded by endless fields of wild garlic (Allium ursinum)

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Wild garlic (Allium ursinum)

Bluebells (Hyacinthoides non-scripta)

Weirdly beautiful, the inflorescence of pendulous sedge (Carex pendula), typical for the wettest spots in the forest

Another one from the wet plots: large bitter-cress (Cardamine amara)

Another one from the wet plots: large bitter-cress (Cardamine amara)

Young beech leaves, as soon as they are fully grown, spring in the understory is over

A beech forest without understory, most likely too dry and too acid for any survivors

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Young beech seedling (Fagus sylvatica)

Bluebells (Hyacinthoides non-scripta)

Bluebells (Hyacinthoides non-scripta)

Bluebells (Hyacinthoides non-scripta)

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Bluebells (Hyacinthoides non-scripta)

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The entrance to the cathedral of Montpellier

The cathedral of Montpellier

The entrance to the cathedral of Montpellier

The cathedral of Montpellier

Narcissus poetics

The cathedral of Montpellier

The botanical garden of Montpellier

The botanical garden of Montpellier

The botanical garden of Montpellier

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Brackish Camargue vegetation

Brackish Camargue vegetation

A typical lagune

Brackish Camargue vegetation

Camargue horses

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Brackish Camargue vegetation

Brackish Camargue vegetation

Brackish Camargue vegetation

Camargue horses

Brackish Camargue vegetation

Little egret in the evening sun

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A typical lagune

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Grass lily

Dandelion fuzz

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Poplar

Gare du Midi, Brussels

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Yellow leaves of mountain birch, with lake Torneträsk in the background.

Lapporten, the gate to Lapland, in Abisko

Rain blowing over the Abisko National Park

The colours of the north: red fireweed and yellow mountain birches, with lake Torneträsk on the background

Yellow leaves of mountain birch, with lake Torneträsk in the background.

Rain on the background, the ski lift in Abisko on the foreground

The steep slope of mount Nuolja on a dramatic looking morning

The beautiful colors of lake Torneträsk in Abisko

A little stream on top of the mountain, with a view on Lapporten, the gate to Lapland

Well, that is a beautiful table with a nice view on lake Torneträsk in Abisko

Our little experiment on top of the mountain in Abisko, with a view on Lapporten

Autumn in Abisko is extremely colorfull

The ski lift with a view on Abisko National Park and Lapporten

Hiking dowhill towards lake Torneträsk

This green is greener than the greenest green: moss on top of mount Nuolja

Well, that is a beautiful table with a nice view on lake Torneträsk in Abisko

The ski lift with a view on Abisko National Park and Lapporten

The ski lift with a view on Abisko National Park and Lapporten

The most beautiful hiking trail of the world: Nuolja in Abisko

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The most beautiful hiking trail of the world: Nuolja in Abisko

Cirsium helenioides, the melancholy thistle

Hiking down mount Nuolja

The steep slope of mount Nuolja on a dramatic looking morning

The colours of the north: red fireweed and yellow mountain birches, with lake Torneträsk on the background

The prettiest yellow and blue: autumn in Abisko

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Campanula or bellflower, I think ‘uniflora’

Vaccinium myrtillus

Cornus suecica, the prettiest red of the world

Hieracium alpinum, alpine hawkweed

Carex atrata, one of my favourite sedges

Alpine clubmoss, Diphasiastrum alpinum

Agrostis capillaris, bentgrass

Common yarrow (Achillea millefolium)

Anthoxanthum odoratum, sweet vernal grass, fully grown and mature

Snow scooter trail

Our plot in the mids of a field of horsetails (Equisetum pratense)

Equisetum pratense

Cliff overlooking the valley with the road to Norway

Seedling of Taraxacum officinale, the dandelion, after two years of growing in bad conditions

Poa alpina, the alpine meadow-grass, with its viviparous seeds

Massive flowerhead of Angelica archangelica

Angelica archangelica

Blueberry (Vaccinium myrtillus) in autumn

A lowland marsh in Abisko in autumn

Installing the plots of our trail observations on top of mount Nuolja

Installing the plots of our trail observations on top of mount Nuolja

Tanacetum vulgare (Tansy), non-native for the high north

Autumn forest down in the valley

The valley of Nuolja to Björkliden

Summer on the Nuolja-side

A full rainbow behind mount Nuolja in Abisko

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A strong wind blowing rain from behind the mountains to our side

A strong wind blowing rain from behind the mountains to our side

Betula nana, the dwarf birch, mini autumn forest

Betula nana, the dwarf birch, mini autumn forest

The valley of Björkliden in autumn

The valley of Björkliden in autumn

The valley of Björkliden in autumn

The valley of Björkliden in autumn

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Rubus arcticus

Eriophorum vaginatum

Silene acaulis

Dryas octopetala

Ranunculus glacialis

Western European species like the red clover (Trifolium pratense) here are often listed as non-native species in mountain regions.

Cornus suecica

Ranunculus glacialis

A rainy hike

Although the alpine zone has been harder for invasives to access than most places, human structures like trails are often an easy gateway for the invaders to get up there. Picture from Abisko, Swedish Lapland.

Oxyria digyna

Trifolium repens

Salix reticulata

Trifolium pratense

Melting snowpatch on a lake

Silene suecica

Bartsia alpina

The valley of the lakes

Overlooking the valley of Laktajakka

Amiens

View from my office window

Colourful mirror

Cathedral at night

The southern side

Winter sun on the Place du Don

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The museum behind the beautiful gates

Enjoying silence and the morning sun

Sunny but cold, the Quai Bélu

Cold!

Almost cold enough for ice-skating

Frozen to the bone

Frozen mirror

Cathedral at night

Le Club d’Aviron in winter weather

Gargoyle planning to eat the cathedral

Just outside of Amiens

Nice architectural curve

Cathedral seen from the frozen Parc Saint-Pierre

House on the square before the cathedral

Sun rising above the water

View from my office window

Cathedral with a glimpse of spring

Cathedral at night

Cathedral at night

Maria without a shirt

Sunny but cold, the Quai Bélu

Sweden autumn 2015

Lichen

Sweden summer 2015

View on the 1000 meter plots

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Plots flooded by the snowmelt

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Snowbridge, maybe don’t cross…

Snowbridge

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Steep slope

Hiking down

Narvik Kirche, church of the subarctic

Narvik Kirche

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Narvik Kirche

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Building a bridge over the fjord will gain al drivers at least an hour